linux-kernel - Re: [PATCH v2] ata: ahci: Do not check ACPI_FADT_LOW_POWER

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Message-ID: <CAJZ5v0in-me017RoR8yRMMXmbBofr6u9o2_WMGh38cpiy3cG3w@mail.gmail.com>
Date:   Thu, 25 Aug 2022 20:26:43 +0200
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     "Limonciello, Mario" <mario.limonciello@....com>
Cc:     "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Damien Le Moal <damien.lemoal@...nsource.wdc.com>,
        "open list:LIBATA SUBSYSTEM (Serial and Parallel ATA drivers)" 
        <linux-ide@...r.kernel.org>,
        Linux ACPI <linux-acpi@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] ata: ahci: Do not check ACPI_FADT_LOW_POWER_S0

On Thu, Aug 25, 2022 at 8:17 PM Limonciello, Mario
<mario.limonciello@....com> wrote:
>
> On 8/25/2022 13:01, Rafael J. Wysocki wrote:
> > From: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> >
> > The ACPI_FADT_LOW_POWER_S0 flag merely means that it is better to
> > use low-power S0 idle on the given platform than S3 (provided that
> > the latter is supported) and it doesn't preclude using either of
> > them (which of them will be used depends on the choices made by user
> > space).
> >
> > For this reason, there is no benefit from checking that flag in
> > ahci_update_initial_lpm_policy().
> >
> > First off, it cannot be a bug to do S3 with policy set to either
> > ATA_LPM_MIN_POWER_WITH_PARTIAL or ATA_LPM_MIN_POWER, because S3 can be
> > used on systems with ACPI_FADT_LOW_POWER_S0 set and it must work if
> > really supported, so the ACPI_FADT_LOW_POWER_S0 check is not needed to
> > protect the S3-capable systems from failing.
> >
> > Second, suspend-to-idle can be carried out on a system with
> > ACPI_FADT_LOW_POWER_S0 unset and it is expected to work, so if setting
> > policy to either ATA_LPM_MIN_POWER_WITH_PARTIAL or ATA_LPM_MIN_POWER is
> > needed to handle that case correctly, it should be done regardless of
> > the ACPI_FADT_LOW_POWER_S0 value.
> >
> > Accordingly, drop the ACPI_FADT_LOW_POWER_S0 check from
> > ahci_update_initial_lpm_policy() along with the CONFIG_ACPI #ifdef
> > around it that is not necessary any more.
>
> Looking at the source commit for this behavior:
>
> b1a9585cc396 ("ata: ahci: Enable DEVSLP by default on x86 with SLP_S0")
>
> It was trying to set a policy tied to when the system is defaulting to
> suspend to idle.
>
> To try to match the spirit of the original request but not tying it to
> the FADT, how about using pm_suspend_default_s2idle()?

The user can switch to "default S3" later anyway, so this wouldn't
help more than the check being dropped.